A power system consists of several electrical components (e.g. generators, transmission lines, loads) connected together, its purpose being generation, transfer and usage of electrical power.
In a conventional On-Line Tap Changer (OLTC) the control is given by a simple integrator with a time delay and deadband. The size of the deadband sets the tolerance for long term voltage deviation. The reference signal for the integrator is the secondary voltage setpoint. This is usually kept constant at the desired secondary voltage.
Voltage stability of a power system is defined by the IEEE Power System Engineering Committee as being the ability of the system to maintain voltage such that when load admittance is increased, load power will increase so that both power and voltage are controllable [2].
Voltage stability in power networks is a widely studied problem. Several voltage collapses resulting in system-wide black-outs made this problem of major concern in the power system community.
In todays state-of-the-art practice, the following methods are used to detect that the system is close to voltage instability:                1. As too much power is requested by the load, the generators will start using their rotational energy, implying that the frequency of the voltage (50/60 Hz) will start to decrease. Detecting a low frequency has been a too slow measure to stop the voltage collapse in for example eastern USA in 2003.        2. Another sign of overload is that the load voltage drops. However, it has been shown that neither this is a good measure for the instability of the grid.        
Using any of the above methods (or similar), the actions taken by the power companies is usually one or both of the following:                1. Connect capacitor banks, to increase the active effect that can be consumed by the load. If this is done in time, a voltage collapse can sometimes be avoided. A disadvantage of this method is that it makes the network more sensible to load variations.        2. Disconnect certain amounts of load (load shedding). This is a very “expensive” measure, and therefore avoided for as long as possible by the power company. However this measure can prevent the whole power net from collapsing.        
This invention is concerned with dynamic stability of a power systems. The inventors propose a dynamic feedback and feed-forward based compensation that aims at stabilization of the power grid. This control structure is intended to function as an emergency control scheme, i.e., it will be active in critical situations when the network is near voltage collapse.